Huntington's Disease (HD) is a fatal neurodegenerative disorder caused by an expanded polyglutamine (polyQ) tract in the protein huntingtin (htt). Sadly, a decade of research into the mechanisms of polyQ- dependent neurodegeneration has failed to produce even a single effective therapy for HD. Although small molecules have been identified that inhibit the aggregation of a mutant htt fragment in vitro, many bind to secondary structures shared by other proteins, and it is not known if any of these molecules will be effective and specific in more complex models of HD. We recently completed a loss-of-function (LOF) genomic screen in S. cerevisiae with single gene deletion strains that identified kynurenine 3-monooxygenase (KMO), an enzyme in the KP of tryptophan degradation, as a potent suppressor of mutant htt toxicity. The brain levels of two neurotoxic metabolites in the KP, quinolinic acid (QUIN) and 3-hydroxykynurenine (3-HK), are increased in the striatum and neocortex in early grade HD; similar increases in QUIN and/or 3-HK are present in three mouse models of HD. QUIN and 3-HK have long been hypothetically linked to the pathophysiology of HD. Indeed, intrastriatal injection of QUIN together with 3-HK causes striatal lesions that may be mediated by the combination of N-methyl D-aspartate (NMDA) receptor over-stimulation (excitotoxicity) and free radical formation. In our proposal, we present data showing that Ro 61-8048, a high-affinity, orally bioavailable small molecule inhibitor of KMO, decreases QUIN, 3-HK and mutant htt toxicity in yeast, and significantly improved neurological index, rotarod performance, locomotor activity and ambulatory distance in a small pilot study using a mouse model of HD. Remarkably, we show KMO is expressed exclusively in microglia. Microglial activation has been documented in postmortem brains of early grade HD patients and in HD mouse models. However, little is known about the role of microglia in HD pathophysiology. We show that primary microglia isolated from HD mice have significantly increased levels of 3-HK. We hypothesize that mutant htt induces a transcriptional defect that activates the KP in microglia, and that inhibiting the KP via pharmacological and genetic approaches will improve behavioral and pathological outcome measures in HD mouse models. We propose to test these hypotheses by studying the role of mutant htt and the KP in cultured microglia and in vivo in mouse models of HD. These experiments will establish whether KMO inhibitors such as Ro 61-4048, which showed promising results in a small pilot study, deserves further consideration for pre-clinical development as a HD therapy. More broadly, our genetic experiments in mice will determine whether pharmacological inhibition of KMO is a bona fide therapeutic approach to treating HD. PUBLIC HEALTH RELEVANCE In this project we will use genetic and pharmacological approaches to determine if blocking a metabolic pathway implicated in Huntington's disease (the kynurenine pathway) confers protection in mouse models of this disorder. The kynurenine pathway is found predominantly in microglia, the macrophages of the brain, which are activated abnormally in pre-symptomatic Huntington's disease patients. If successful, our studies may lead to clinical tests of small molecule inhibitors of the pathway in patients with Huntington's disease.